There have been used spark plugs for igniting a fuel mixture gas in an internal combustion engines mounted on motor vehicles. Such a spark plug has a structure in which a central electrode and a ground electrode are arranged facing with each other along an axial direction of the spark plug. A spark plug is mounted on an engine head of an internal combustion engine so that a front end part of the spark plug faces an inside of a combustion chamber of the internal combustion engine. A spark discharge is generated at a discharge gap between the central electrode and the ground electrode of the spark plug so as to ignite a fuel mixture gas in the combustion chamber. There is room for improvement on a structure of a spark plug so as to improve ignition capability of the spark plug to ignite a fuel mixture gas.